This application is based upon and claims the benefit of Japanese Patent Application No. 2000-220913 filed on Jul. 21, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
This invention relates to a dynamic quantity sensor including a movable portion, a movable electrode integrated with the movable portion and a fixed electrode arranged opposite to the movable electrode for detecting a dynamic quantity based on a change of an interval between the movable electrode and the fixed electrode.
2. Description of the Related Art
JP-A-11-326365 discloses a capacitance-type semiconductor dynamic quantity sensor. In this dynamic quantity sensor, a movable portion is connected to a base portion through a spring portion so as to move in a predetermined direction. Further, movable electrodes are integrated with the movable portion, and fixed electrodes are arranged opposite to the movable electrodes. When a dynamic quantity is applied to the sensor, the sensor detects the applied dynamic quantity by detecting changes of intervals between the movable electrodes and the fixed electrodes as capacitance changes.
In the conventional dynamic quantity sensor, however, since the movable portion is connected to the base portion through the spring portion, when a large shock is applied to the sensor, the movable portion performs free vibration (oscillation) for awhile even after the shock disappears. If the free-vibration time becomes longer in this manner, the intervals between the movable electrodes and the fixed electrodes fluctuate due to the free-vibration. Therefore, even after the large-shock application is ended, signals would be output from the sensor as if the shock were still applied to the sensor, thereby degrading the sensitivity of the sensor output.
The present invention has been made in view of the above problem. An object of the present invention is to shorten a free-vibration time of a movable portion so as not to adversely affect sensor output when a large shock is applied to a sensor.
According to the present invention, a dynamic quantity sensor includes a base portion, a spring portion connected to the base portion, a movable portion connected to the spring portion, a movable electrode integrated with the movable portion, and a fixed electrode supported by the base portion. The spring portion moves in a predetermined direction in accordance with a dynamic quantity applied thereto, and the movable portion and the movable electrode move together with the spring portion in the predetermined direction. In this sensor, a Q value of vibration of the movable portion in the predetermined direction is smaller than {fraction (1/500)} of a resonance frequency of the vibration of the movable portion in the predetermined direction.
Accordingly, when a large shock is actually applied to the dynamic quantity sensor, free-vibration time of the movable portion can be made shorter so as not to affect sensor output. Here, the Q value represents a resonance sharpness, and is indicated by xcfx80/xcex4 where xcex4 is a logarithm damping degree of damping vibration.